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by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Oracle’s Table CACHE Option

When Oracle manages I/O against the database, it uses different rules about where new data blocks are placed in the buffer cache. For all I/O’s except full-table scans, rows are read into the most-recently-used section of the Oracle buffer pool. As new data blocks are fetched, the older blocks work their way down to the least-recently-used end of the buffer, where they are eventually erased from the buffer to make room for newly acquired data blocks. (Figure 8.2)

Figure 8.2 Aging blocks from the Oracle buffers

The exception to this rule are data blocks that are acquired by using full-table scans. As data blocks are read into the buffer, blocks are placed on the opposite end of the buffer, in the least-recently-used option of the buffer. In this fashion, full table scans will not interfere with buffers on the most-recently-used end of the buffer (i.e. rows from non-full-table-scan transactions). Since these rows are already at the least-most-recently used end of the buffer, they will be flushed quickly as new rows are fetched as part of the full-table scan. (Figure 8.3) As we may know, full-table scan data blocks are physically read into Oracle’s buffer in chunk sizes that are specified by the init.ora parameter DB_FILE_MULTIBLOCK_READ_COUNT. For example, assume that the DB_BLOCK_SIZE is set to 8192 bytes (8K) and the DB_FILE_MULTIBLOCK_READ_COUNT is set to 8. When Oracle detects a full-table scan, Oracle will perform reads of four physical blocks at a time, pulling in 64K with each I/O. Remember, physical I/O is very time consuming, and anything that can be done to reduce I/O will improve Oracle performance.

Figure 8.3 Different ends of the buffer cache may be used for different tables

This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:


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